1. Field of the Invention
The present invention relates to a lens focus correction device for use with a projection type image display apparatus or the like.
2. Description of the Related Art
As a projection type image display apparatus, there has hitherto been known a projection type television receiver, i.e., so-called rear projector in which an image obtained from a cathode ray tube is projected onto a transparent type screen from the rear surface of the screen in an enlarged scale.
FIG. 1 of the accompanying drawings shows a schematic arrangement of the rear projector. In FIG. 1, reference numeral 1 depicts a cabinet and reference numeral 2 depicts a three-tube color cathode ray tube 2 (2R, 2G, 2B) corresponding to red (R), green (G) and blue (B) disposed within the cabinet 1. An image from the color cathode ray tube 2 is magnified by a projection lens 3, reflected by a reflection mirror 4 and then projected onto a front screen 5 from the rear surface thereof.
FIG. 2 shows an example of the rear projector cathode ray tube 2 to which the projection lens 3 is integrally attached. The cathode ray tube 2 uses a liquid-cooling tube in which liquid-cooling liquid 8 is filled into a space formed by a panel front surface, a curvature of field correction lens 7 and a sealing frame 9. A tube 10 constructing the projection lens 3 is fixed to the sealing frame 9 of the liquid-cooling liquid 8 by some suitable means, such as screws 11 or the like.
The tube 10 is composed of an outer tube 12 and an inner tube 13 rotatably disposed within the outer tube 12 so that the inner tube 13 can be moved along an optical axis. The inner tube 13 has a plurality of optical lenses 15, 16 and 17 arrayed therein. In this example, the four optical lenses 15, 16, 17 and 7 constitute a projection lens, i.e., main lens group 3.
The outer tube 12 has on its outer peripheral wall portion an oblique groove, i.e., oblique guide slot 18 defined with a predetermined angle relative to an optical axis C from a plane standpoint. The inner tube 13 has a guide member 19 integrally extended therefrom. The guide member 19 is engaged with the oblique guide slot 18.
The guide member 19 has a thumbscrew 22 fitted thereto through a spacer 23 in order to fix the inner tube 13 after the inner tube 13 was set at a predetermined position with respect to the optical axis C direction.
The inner tube 13 has on its front and rear ends disposed sliding portions 20 to come in contact with the inner peripheral surface of the outer tube 12 to thereby slidably support the inner tube 13.
The outer tube 12 and the inner tube 13 are each made of plastics (resin).
In this tube 10, when the initial focus position of the main lens group 3 is set, initially, the thumbscrew 22 is loosened and the guide member 19 is moved along the oblique guide slot 18, whereby the inner tube 13 is displaced in the optical axis C direction while the inner tube 13 is being rotated with respect to the outer tube 12, thus adjusting the lens focus position. After the lens focus position has been adjusted, the inner tube 13 is fixed to the outer tube 12 at that position by means of the spacer 23.
In the above-mentioned rear projector, a heat generating source, such as the cathode ray tube 2 and peripheral circuit portions and the optical system, such as lens or the like, i.e., the tube 10 are disposed close to each other. Thus, as shown in FIG. 3, as a temperature of the rear projector equipment rises after the cathode ray tube set has been energized, respective portions (the tube 10 including the lens system or the like) expanded so that the initial focus position of the main lens group 3 is changed which deteriorate an image.
As a means for correcting the lens focus position changed due to a rise of temperature, there is the following method. According to this method, when a lens is designed, a plastic lens whose volume changing ratio with a temperature is large is divided by a plurality of plastic lenses to weaken an optical power (i.e., refracting power) per plastic lens and an optical power of a glass lens whose volume changing ratio is small is strengthened to thereby suppressing the amount in which the focus position is changed. As examples of the above-mentioned technique, there are design methods disclosed in U.S. Pat. No. 4,776,681, Japanese laid-open patent publication No. 61-205909 and SID, 1/1, 1933 (A46-in, high-resolution rear-Projection display) or the like.
In the example shown in FIG. 2, in the main lens group 3 comprised of the four lenses, most of positive power of the optical system is produced by the second optical lens, i.e., the glass lens 16 of which the volume changing ratio is small, thereby decreasing the optical powers of the third and fourth optical lenses 15, 17 and 7 made of plastics.
However, even though the volume changing ratio of the lens itself is reduced, most of the tube 10 for supporting the lenses is made of a resin material and the change of the focus position due to the volume change of the tube 10 was never taken into consideration.
In the case of FIG. 2, the outer tube 12 provided between the cathode ray tube 2 and the guide member 19 is thermally expanded due to the rise of temperature so that the main lens group 3 is moved to the left-hand side direction in FIG. 2. As a result, the focus is displaced from the initial focus position and an image is deteriorated.